As is well-known, the conventional thermostatic mixing valves typically include a thermostatic control element of the so-called "wax" type wherein thermally expansive paraffin wax composition is charged. The control element is adapted to feedback control the position of a mixing valve member in response to the temperature of a mixture of hot and cold water in such a manner that a mixture having a desired temperature is provided. When fluctuation occurs in the supply line pressures, supply temperatures or flow rates, causing a change in the mixture temperature, the heat sensitive wax composition expands or contracts in response to such temperature change to automatically displace the valve member. After repeating a number of cycles of overshooting and undershooting, the valve member will finally be adjusted to a new position wherein the actual mixture temperature is brought close to the desired temperature.
Generally, the paraffin wax composition used in the thermostatic control elements is adapted to exhibit a volumetric change due to phase transformation between the solid and liquid phases. Because such change in phase necessarily involves absorption and evolution of the latent heat, in addition to transfer of the sensible heat, the heat capacity thereof is considerably large. Moreover, the paraffin composition has a poor thermal conductivity. Accordingly, the disadvantage of the wax type thermostatic element is that the responsiveness thereof is limited. It has often been observed that, once transitional change has occurred in water temperature for any reasons, several seconds of overshooting and undershooting of temperature of an intolerable magnitude unavoidably last in the conventional mixing valves before the delivery of water having the desired temperature is resumed.
In order to improve the responsiveness, use of a thermostatic control element made of a shape memory alloy such as an alloy formed of nickel and titanium has been proposed in the prior art (see, e.g., Japanese Utility Model Kokai Publication No. 58-11177; Japanese Patent Kokai Publication No. 58-24669; Japanese Utility Model Kokai Publication No. 58-187666; Japanese Utility Model Kokoku Publication No. 61-44062; Japanese Utility Model Kokoku Publication No. 61-23987; Japanese Utility Model Kokai Publication No. 61-150585). These references generally refer to the shape memory alloy (SMA) as being a metal having a shape memory effect wherein the shape of an article deformed below the martensitic transition temperature of the SMA tends to spring back to the initial shape imparted in the austenitic mother phase when heated above the transition temperature. It is believed, however, that the property of the SMA that the modulus of elasticity thereof varies with temperature is more important in its application to the thermostatic control elements.
The prior art cited above discloses a thermostatic mixing valve having a coiled spring made of the SMA. Throughout various references, a valve member controlling the flow of hot and cold water is supported between the coiled spring made of the SMA and another coiled bias spring made of the conventional spring steel. The valve member is held in such a position that the spring force of the SMA spring and that of the bias spring are balanced. During transitional condition wherein the mixture temperature is altered, the spring force of the SMA spring varies in response to the temperature change, causing the valve member to move until the balance is resumed between the SMA and bias springs. Some of the references propose to provide an adjusting handle adapted to alter the preload either of the SMA spring or the bias spring to enable the user to adjust the desired mixture temperature.
Obviously, the use of the SMA coiled spring as the temperature responsive thermostatic control element is advantageous in providing an improved responsiveness of the mixing valve since the SMA is metallic and therefore has a small heat capacity and an increased thermal conductivity as compared with the conventional paraffin wax composition.
Notwithstanding the advantage thereof mentioned, however, it appears that, hitherto, a commercially feasible thermostatic mixing valve employing the SMA coiled spring has never been realized.
The problem associated with the coiled springs made of an SMA such as a nickel and titanium alloy is that, currently, those SMA springs which can develop an adequately strong spring force enough to control the valve member of the mixing valve for a sufficiently long period commensurate with the service life of the mixing valve are not commercially available.
The primary reason is that they are subject to thermomechanical fatigue when operated under excessively loaded conditions. More specifically, the modulus of elasticity of the SMA is prematurely degraded if subjected to hot-and-cold heat cycles under an excessive strain. Application of excessive strain would result in an earlier fatigue of the SMA coiled spring so that the spring force obtainable thereby is prematurely decreased during heat cycles. Although the mechanism of the degradation in the modulus of elasticity of the SMA or the thermomechanical fatigue of the SMA spring is not known with any degree of certainty, it is believed that the number of crystals which are involved in the martensitic transformation is decreased with time when the SMA coiled spring is excessively deformed. To avoid premature fatigue, the coiled spring of SMA must not be subjected to excessive strain. It is to be noted that the amount of strain permissible for the commercially available SMA springs is extremely small as compared with a coiled spring made of the ordinary spring steel. As the spring force developed by a coiled spring is proportional to the amount of strain (degree of deformation) caused by a stress (load) applied to compress the spring, only a limited spring force will be available if it is to be used under a limited strain condition.
A secondary reason is that the SMA coiled springs formed from an SMA wire having such a diameter large enough to develop a strong spring force are difficult to manufacture and, therefore, are presently extremely expensive.
Accordingly, it is desirable to use a small sized SMA coiled spring and to operate it in such a manner as to develop as small spring force as possible.
These requirements, however, give rise to other problems which must also be overcome in designing commercially feasible thermostatic mixing valves employing the SMA spring.
First, in contrast to the conventional thermostatic mixing valve wherein the flow control valve member is relatively rigidly and forcibly positioned by the wax type thermostatic control element designed to develop a thrust in the order of 15 kg and by an adequately strong return spring having a spring force of 4-5 kg, the flow control valve member in the mixing valves incorporating the SMA spring is resiliently positioned between the SMA coiled spring and the bias coiled spring which are acting on the valve member in the opposite directions. In the balanced position, the resultant force of these springs is zero so that the valve member is in a pressure sensitive condition. Therefore, when there exists or occurs a pressure difference between the hot and cold water, the valve member will readily be thrust due to the hydraulic force if the SMA spring is designed and arranged to exhibit a limited spring force. This would result in the deviation or offset of the mixture temperature from the desired value.
Second, the valve member is more or less subject to frictional resistance which resists the sliding movement of the valve member. Each time the valve member is to be moved to a new position to perform its temperature control function, the SMA spring or the bias spring must overcome the frictional resistance. Such frictional resistance brings about a hysteresis loop in the temperature vs. spring force curve. This also leads to the offsetting of the mixture temperature. Smaller the spring force of the SMA spring is, the effect of the frictional resistance becomes greater and not negligible.